1. Field of the Invention
The present invention relates to a liquid crystal display device, and, in particular, to a liquid crystal display device having a countermeasure to a light leakage in a dot-inversion driving method or in a column inversion driving method.
2. Description of the Related Art
In a liquid crystal display device, a first substrate on which pixel electrodes, thin film transistors (hereinafter referred to as “TFT”), etc. are formed in a matrix form and a second substrate on which color filters or the like are formed at positions corresponding to the pixel electrodes oppose each other, and liquid crystal is provided between the first substrate and the second substrate. A transmittance of light by the liquid crystal molecules is controlled for each pixel, to form an image.
Over the first substrate, a video signal line which extends along a vertical direction and which is arranged along a horizontal direction and a scan signal line which extends along the horizontal direction and which is arranged along the vertical direction are provided, and a pixel is formed in a region surrounded by the video signal line and the scan signal line. The pixel primarily comprises a pixel electrode and a thin film transistor (TFT) which is a switching element. A display region is formed with a plurality of pixels formed in a matrix form.
Driving methods of the liquid crystal display device include a frame inversion driving method, a line inversion driving method, a dot inversion driving method, a column inversion driving method, etc. Of these methods, the dot inversion driving method is superior in image quality, and, thus, is used in a monitor or a television of middle or greater sizes. Among liquid crystal display modes, a TN method is most widely in use. In the liquid crystal display element of TN method, positive type liquid crystal molecules aligned parallel to the substrate are twisted by 90 degrees between two opposing substrates, and the alignment of the liquid crystal molecules are switched between horizontal and vertical with respect to the substrate by switching an electric field ON and OFF, so that the optical rotation state of the light is changed and brightness and darkness are displayed. Although the TN-type devices are easily manufactured, there is a disadvantage such as a narrow viewing angle.
In the TN method, on the other hand, a phenomenon of disturbance of the alignment of the liquid crystal or a reverse tilt domain may occur at the periphery of the pixel electrode. Such an alignment disturbance or reverse tilt domain leads to light leakage from a backlight, resulting in a reduction in contrast. As a countermeasure for these phenomena, there are known JP 2004-246280 A and JP 10-104664 A. In addition, JP 2004-348130 A discloses that, in VA-type liquid crystal display devices in which the liquid crystal is aligned in a vertical direction of the substrate, alignment of the liquid crystal at the periphery of the pixel may be disturbed and texture and light leakage may occur because of an electric field in the horizontal direction which is generated between adjacent pixels sandwiching the video signal line.